This invention relates to a method of detecting the degree of corrosion of metal materials, and particularly to a method of detecting the degree of corrosion of metal materials of which power plants, chemical plants and so on are made or of metal materials of which semiconductors, thin films, wiring conductors and so on are made in the information industry.
The method of detecting the intergranular corrosion is variously described in "ANTICORROSION TECHNOLOGY HANDBOOK" (edited by ANTICORROSION ASSOCIATION, and published by NIKKAN KOGYO SHINBUNSHA). For example, there are the oxalic acid etching test for determining the intergranular corrosion resistance from the etched texture caused by electrolytic etching in a 10-% oxalic acid solution, the sulfuric acid/ferric sulfate corrosion test for determining the degree of corrosion from the weight loss after the object being tested is immersed in a boiled 50-% sulfuric acid+ferric sulfate solution for 120 hours, the nitric acid/fluoric acid corrosion test for determining the degree of corrosion from the weight loss after the object being tested is immersed in a 10-% nitric acid +3-% fluoric acid solution at 70.degree. C. for two hours, the nitric acid corrosion test for determining the degree of corrosion from the weight loss after the object being tested is immersed in a boiled 65-% nitric acid for 48 hours, the sulfuric acid/copper sulfate corrosion test for determining the degree of corrosion from the presence or absence of cracks after the object being tested is immersed in a boiled 15.7-% sulfuric acid+5.5-% copper sulfate+pieces of copper solution for 16 hours and bent for opening processing, and the electrochemical potentiokinetic reactivation test (EPR method) for determining the degree of corrosion from the ratio of going and returning peak currents which can be obtained by the measurement of the going and returning polarization curves in a 0.5-M sulfuric acid+0.01-M potassium thiocyanate solution at 30.degree. C. These tests are standardized by JIS (Japanese Industrial Standard).
Of these tests, the EPR is a nondestructive evaluation method as reported in "ANTICORROSION TECHNOLOGY" 29, pp. 163-169 (1980) and is excellent over the other tests because the determination of sensitization degree can be made in a relatively short time. This test, for these reasons, is used for detecting the corrosion of materials of which an actual plant is made.
The EPR is the method of interdetecting the intergranular corrosion, particularly the degree of sensitization, or the phenomenon in which the chromium concentration is reduced in the grain boundary due to formation of the chromium carbide which is produced by the segregation of carbon into the grain boundary when stainless steel or the like is heated. In the EPR, a metal sample of which the corrosion degree is desired to evaluate is immersed as electrodes in a volume, 200 cm.sup.3 or above of a 0.5-M sulfuric acid +0.01-M potassium thiocyanate solution at 30.degree. C. Then, the sample is applied with a range of potential from the corrosion potential to 0.300 V vs. SHE scanned in the master or anodic direction and immediately back to the corrosion potential in the slave or cathodic direction at a linear polarization speed of 100.+-.5 mV/min as an electrochemical measurement condition. The degree of sensitization is determined by the ratio between the current flowing at the time of voltage application in the anodic (going) direction and the current flowing at the time of voltage application in the cathodic (returning) direction, or by the reactivation rate as expressed as follows. ##EQU1##
Thus, the EPR needs a volume of solution cell, 200 cm.sup.3, measuring time of 5 minutes, and an amount of electricity, 10 through 15 C/cm.sup.2.